dr update should end with a digest of what happened

[pazdera]

Add a pretty digest of which packages were rebuilt during a dr update, from which revision to which and whether the build was passed or failed.

Nobody really reads the whole log anyway, so this feature could be really useful.

Example format

$ dr build
... doing stuff ...
dr build    make-pong      1a7ccde -> 87a7aa7  PASSED
dr build    make-music     no changes          SKIPPED
dr build    make-minecraft ed1cdec -> c4a4ac7  FAILED
dr build    Summary: 1 rebuilt, 1 failed, 1 skipped.

### What needs to be done

1. Modify the `dr update` [subcommand](https://github.com/KanoComputing/kano-repository-manager/blob/master/bin/dr#L456) to keep track of errors and build revisions, so they can be printed at the end of the build.